A motor vehicle transmission generally includes a number of gear elements coupling an input shaft and an output shaft, and also includes a number of torque-transmitting devices or clutches that may be selectively engaged to activate certain gear elements and establish a desired speed ratio between the input and output shafts. As used herein, the term “torque-transmitting device” will be used collectively to refer to all clutches, including braking clutches as well as rotating clutches.
Shifting from one speed ratio to another is performed in response to engine throttle level and vehicle speed, and generally involves releasing or disengaging a clutch associated with the current speed ratio, i.e. the off-going clutch, and applying or engaging a clutch associated with a desired new speed ratio, i.e. the on-coming clutch. The term “speed ratio” is defined as the transmission input speed or torque converter turbine speed divided by the transmission output speed. Thus, a low gear range has a high speed ratio while a high gear range has a lower speed ratio.
A shift made from a high speed ratio to a lower speed ratio is referred to commonly and herein as an “upshift”. In the type of transmission involved within the scope of this invention, such an upshift is accomplished by disengaging a clutch associated with the higher speed ratio and engaging a clutch associated with the lower speed ratio, to thereby reconfigure the gear set to operate at the lower speed ratio. Shifts performed in the above manner are termed “clutch-to-clutch” shifts, and require precise timing in order to achieve optimal quality shifting, and to tend to reduce a perceptible delay in the shift event.
Conventional transmissions use various compliance devices such as accumulators, wave plates, springs, and orifices, as well as an applied hydraulic line pressure, to control the shift event. The quality of a particular shift event, and in particular the minimization of a delay in executing the shift, depends on rapid cooperative operation of several clutch functions, such as pressure changes within the respective apply chambers of the on-coming and off-going clutch apply chambers, and the timing and control of the various compliance devices.
In clutch-to-clutch systems, single clutches may perform multiple clutch functions. For example, one clutch may handle low torque, closed-throttle downshifts while remaining capable of handling a high torque upshift. A high torque upshift may require rapid pressurization or fill of the clutch chamber while overcoming compliance to rapidly initiate a desired speed ratio change, which in some circumstances may lead to less than optimal upshift delays.